The present invention relates to an apparatus as and techniques directed to actively determining the power requirements of an acoustic ink printhead and supplying this information to an RF power supply which powers the printhead.
In acoustic ink printing an array of ejectors, forming a printhead, is covered by pools of liquid ink. Each ejector can direct a beam of sound energy against a free surface of the liquid ink. The impinging acoustic beam exerts radiation pressure against the surface of the liquid. When the radiation pressure is sufficiently high, individual droplets are ejected from the liquid surface.
The ejectors may be arranged in an array of rows and columns, with the rows stretching across the width of the recording medium, and the columns of ejectors approximately perpendicular along the movement of the recording media.
Ideally, when each ejector is activated it ejects a droplet identical in size and velocity to the droplets of all the other ejectors in the array. Thus, each ejector should operate under identical conditions.
As may be appreciated, acoustic printing is subject to a number of manufacturing variables including thicknesses and stresses on the ultrasonic transducers, electromagnetic reflections on transmission lines, variations in acoustic coupling efficiencies, and variations in the components associated with each transducer. These variables, if not controlled sufficiently, result in non-uniform droplets, i.e. droplets that vary in size, ejection velocity, and/or other characteristics. Non-uniform droplet size produces undesirable intensity variations in the final image, while non-uniform ejection velocity produces mis-aligned droplets. Non-uniform droplets may degrade the desired output, such as an image, so much that it becomes unacceptable.
Further, even if an acoustic printhead is designed to eliminate negatively impacting manufacturing variables do, control of input power to the printhead may be needed. However, when the manufacturing variables exist, there is less margin for power variability, and therefore, precise power control is desirable. Thus, one manner of improving uniformity in droplet ejection is to ensure that power supplied to a row of ejectors is consistent for each of the ejectors. However, due to manufacturing and other variables the power supplied at one end of a row is known to vary. Therefore, an ejector close to the RF source may receive a higher level of energy than one located distant therefrom.
One proposal to provide balanced power to a row of ink ejectors is by the use of what is known as xe2x80x9cdummyxe2x80x9d transducers associated with transducers designed to emit a droplet. Under this architecture, the same amount of power is consistently supplied to a row of ejectors. If a particular ejector is not to be activated, the power is passed to the dummy transducer rather than to the transducer which would cause ejection. This arrangement allows for a consistent application of power to a row of ink ejectors, no matter how many ejectors are being fired in a row of the printhead. However, a drawback to this configuration is the amount of area on the printhead necessary for the xe2x80x9cdummyxe2x80x9d transducers. Additionally, whether one or all ejectors (i.e. 128) are used, 100% power is supplied to the row. This results in wasted energy, and causes an undesirable rise in printhead temperature.
Another design, used to obtain droplet uniformity, is described in U.S. Pat. No. 5,389,956, which provides techniques for improving uniformity by compensating for row-to-row variations in the average droplet uniformity by row-wise control of the electric power applied to the transducers. The power applied to each row is adjusted so as to achieve uniform average droplet characteristics from each row. This technique also uses an impedance matching network to match its input to the impedance of the printhead components.
Another technique described in the above patent is to vary the efficiency of the individual droplet ejectors by physically trimming (such as with a laser) one or more of their associated components. Specifically, this may be accomplished by physically trimming the dimensions of the individual transducers, varactors, one or more resistors, or one or more capacitors. Components may be included in the basic droplet ejector specifically for trimming.
Another technique described in U.S. Pat. No. 5,389,956 is to control the voltage applied to the varactors of droplet ejectors. By adjusting the varactor voltage applied to each column (row) as a function of that column""s (row""s) average droplet characteristic, uniform average droplets can be produced by each column (row). Alternatively, by controlling the varactor voltage applied to each droplet ejector that is ejecting a droplet, substantially uniform droplets can be achieved from each droplet ejector. Beneficially, the varactor voltages are obtained via digital-to-analog (D/A) converters controlled by memory devices that store the proper codes for the D/A converters to produce their required voltages.
Existing systems therefore use a passive power balancing network, or use static information to vary the amplification (or the attenuation) of the power amplifier and RF driver used to supply energy to the printhead.
Thus, it is considered desirable to provide an active control network which is external to the printhead to determine the amount of energy required for operation of ink ejectors based on the number of ink ejectors which are going to be used by an active row.
In accordance with the present invention there is provided an apparatuses and techniques to ensure that sufficient power is supplied to an acoustic ink printhead which has variations in impedances during operation, where the impedance variations result due to variations in the number of ejections being used. One manner of ensuring sufficient power is to implement an active controller external to the printhead, which determines an amount of power that is to be supplied to a row of ejectors based on the number of ejectors in an active row which are going to be used. The active controller sends a power supply control signal to the power supply, whereby the power supply control signal determines the amount of power supplied to the row of ink ejectors.
Another aspect of the present invention is generation of a first power supply control signal for a first set of ink ejectors of a row and at least one other power supply control signal for at least one other set of ink ejectors in the same row. The first and at least one other power supply control signals being used in generating an appropriate output signal to the row of ink ejectors when the row is powered.
With attention to yet another aspect of the present invention, the active controller is a forward looking device which makes its determination as to required power for a particular row of ejectors, prior to activation of the ink ejectors for the particular row.
With attention to yet another aspect of the present invention, when a maximum allowable power value is exceeded, an inhibit signal inhibits supplying of power to the printhead, above the stored maximum value.
Turning to yet another aspect of the present invention, the active controller determines where in a row the ink ejectors are to be fired and uses this information in the determination of the amount of power that is to be supplied to the row.
Still yet another aspect of the invention incorporates heat characteristics of the ink ejectors when generating the power control signal.
A principal advantage of the present invention is to ensure a proper supply of power to an active row of ink ejectors to a printhead whose impedance varies, while not requiring 100% output for all instances of ejector activation.
Another advantage of the present invention resides in the elimination of (dummy) transducers, switching elements and associated circuitry to maintain a consistent impedance of the printhead.
Still another advantage of the present invention resides in an active controller, external to the printhead, which increases the accuracy of the amount of power which is required, by determining requirements based on the number of ejectors which are going to be fired. The position of the ejectors which are going to be fired also needs to be taken into consideration.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiment.